As designed Signature option when no signatures is allowed

You can prevent someone from creating a signature, but there are no global permissions over who can see signatures. Even if you have no users who have signatures, an admin could still manually add a signature. So, really, there's no criteria that can be used to decide whether or not to show this checkbox.

If it's really something you don't want, you'd have to remove it from the template yourself.
 
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